Introduction to Switching

 1. Collision Domains and CSMA/CD 

What is a collision domain? When shared media is connected each other using Ethernet and multiple devices are sending data simultaneously Collins happen because sending and receiving is not supported at the same time which cause the loss of data nd time associated with collisions. Therefore, Carrier Sense Multiple Access with Collision Detection (CSMA/CD) was developed. Before sending data, a host will check to see if any other host is already sending data. 


2. Broadcasting

When a Hub receives a broadcast it sends it to every single port on the Hub. Bridge will resolve the issue with collision domain but it will not segment the broadcasting. However, switch can segment the broadcast. 

3. MAC Address table 

Command: show mac address -table or show mac address dynamic. If you need help for the command, type show mac address -table ?

Mac address table is used to help us make a decision on what to do with the incoming frames. It will forward, filter or flood. 


Forwarding happens when the MAC table finds the destination address in the MAC table. Flooding occurs when the switch has no entry for the frame's destination in its MAC address table. Filtering happens when the source and destination MAC address are located off the same port. 

(Note: Broadcast frames are intended for all hosts, and they have a destination MAC of all Fs (ff-ff-ff-ff-ff-ff and they are treated as flooded frame.)

4. Dynamic MAC table command

Type show mac address dynamic

conf t

When one port is down, just move to a different port. 

SW1# int fast 4/0/24

5. Hex Conversions

Hex is not using unit of 1, 10, 100.... Instead, it uses units of 16. So, it is 1, 16, 256...

To express 10, 11, 12, 13, 14, 15 Hex uses letters.

A=10

B=11

C=12

D=13

E=14

F=15

Task: Convert the hex value 3A to decimal. 

3x16+10=58

Task: Convert the decimal 142 to hex. 

142/16=8 with a remainder of 14

8E






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